1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier or a printer that uses an electrophotographic method and includes an optical scanning apparatus that performs optical writing with respect to a photosensitive member.
2. Description of the Related Art
Normally, an image output portion of an image forming apparatus such as a copier or a printer that uses an electrophotographic method carries out image formation by an electrophotographic process that forms a toner image by scanning the surface of a photosensitive member with a laser beam that flickers in accordance with print data, and developing an electrostatic latent image formed on the photosensitive member. In general, an optical scanning apparatus is used for scanning a photosensitive member with a laser beam. An optical scanning apparatus converts luminous flux from a semiconductor laser that is a light source into substantially parallel luminous flux, deflects the luminous flux using a rotating polygonal mirror that rotates, and thereafter causes the luminous flux to imaged in the form of a spot on a photosensitive member through an element of an imaging optical system such as a lens or a mirror.
In the following description, the term “main-scanning direction” refers to a direction that is perpendicular to a rotation axis of a rotary polygon mirror and an optical axis of an imaging optical system (direction in which a laser beam deflected by the rotary polygon mirror scans a photosensitive member). The term “sub-scanning direction” corresponds to a direction that is parallel to the rotation axis of the rotary polygon mirror or a rotational direction of a photosensitive member. The term “main-scanning cross section” refers to a plane that includes the main-scanning direction and the optical axis of the imaging optical system. The term “sub-scanning cross section” refers to a cross section that is perpendicular to the main-scanning cross section.
In recent years, in response to demands to increase the speed of image formation, image forming apparatuses are known that use a light source that emits a plurality of laser beams in an optical scanning apparatus. In particular, since a vertical cavity surface emitting laser (hereunder, referred to as “VCSEL”) facilitates formation of a large number of light emitting points into an array, a large number of optical scanning apparatuses that use a VCSEL have been proposed.
The aforementioned kinds of optical scanning apparatuses have a configuration that controls a light amount of a laser beam that is emitted from a VCSEL. Unlike an edge emitting laser, the emission direction of laser beams emitted from a VCSEL is a single direction. As a configuration for detecting the light amount of a laser beam emitted from the VCSEL, a configuration is known that splits a laser beam emitted from the VCSEL into a plurality of laser beams using a beam splitter or the like that is disposed between the VCSEL and a rotary polygon mirror, and in which an optical sensor receives a laser beam obtained by the aforementioned splitting of the laser beam by the beam splitter. The image forming apparatus controls the light amount of a laser beam that the VCSEL emits based on the light amount of the laser beam received by the optical sensor.
A VCSEL has a characteristic such that a spreading angle (FFP) of a laser beam emitted from the VCSEL changes with a change in the driving current. Therefore, if an aperture is provided between a beam splitter and a rotary polygon mirror, a ratio between a light amount of a laser beam obtained when a laser beam is split by a beam splitter that is detected using an optical sensor and a light amount of a laser beam that passes through the aperture and is irradiated onto the photosensitive member changes, and highly accurate light amount control cannot be performed.
For example, in Japanese Patent Application Laid-Open No. 2002-040350, an optical scanning apparatus is proposed that, after shaping a laser beam using an aperture, splits the light beam with a beam splitter and guides a laser beam obtained by the aforementioned splitting to an optical sensor to detect the light amount. According to this configuration, even if a spreading angle at which light is emitted changes due to a change in the driving current, because the laser beam is split at the beam splitter after the laser beam has been shaped by the aperture, a ratio between a light amount that is reflected by the beam splitter and detected by the optical sensor and a light amount that arrives at the photosensitive member is constant. As a result, light amount control can be performed with high accuracy.
For example, in Japanese Patent Application Laid-Open No. 2006-259098, an optical scanning apparatus is proposed in which an aperture and a beam splitter are integrally formed with each other. According to this configuration, a risk of the positional relationship between the aperture and the beam splitter changing is eliminated, and the positional accuracy can be improved and the number of components can be reduced.
It is known that in an image forming apparatus that forms an electrostatic latent image on a photosensitive member using a plurality of laser beams, the imaging positions of respective laser beams on the photosensitive member deviate in the main-scanning direction, and main scanning jitter arises whereby the amount of deviation thereof differs according to a position in the main-scanning direction. FIG. 6 is a configuration example of a multi-beam scanning system in which two light emitting portions A and B (hereunder referred to as “A laser” and “B laser”) are disposed so as to incline at an angle δ with respect to the main-scanning direction. FIG. 7A shows a state in which light beams emitted from the A laser and the B laser that are the two light emitting portions shown in FIG. 6 image spot images on a photosensitive member, in which a beam from the A laser is indicated by a solid line and a beam from the B laser is indicated by an alternate long and short dash line. In FIG. 7A, the beams emitted from the A laser and the B laser that are light emitting portions intersect at an aperture 207 and are incident on a rotary polygon mirror 210 at points that are separated by a deflection point interval L2. After being deflected by the rotary polygon mirror 210, the beams emitted from the A laser and B laser pass through imaging lenses 221 and 222 and image spot images at positions that are separated from each other in the main-scanning direction on the photosensitive member 82. As shown in FIG. 7A, misalignments La, Lb and Lc in the main-scanning direction arise at the spot images that are imaged on the photosensitive drum 82 by the respective beams emitted from the A laser and B laser. It is possible to correct the misalignments in the main-scanning direction on the photosensitive member 82 by altering the light-emitting timing of the A laser and B laser. However, since the misalignment intervals La, Lb and Lc differ respectively depending on the respective positions in the main-scanning direction (in FIG. 7A, the interval Lb is larger than the interval Lc, and the interval La is larger than the interval Lb), it is not possible to correct all of the misalignments at the same time, and therefore main scanning jitter occurs.
If the photosensitive member 82 has an eccentric component, in some cases the photosensitive member 82 becomes decentered during rotation and moves from a position 82 indicated by a solid line to a position 82′ indicated by a dashed line. The misalignment amount in the main-scanning direction in this case is, for example, an interval La′ at the position corresponding to the interval La, and thus the misalignment amount increases relative to the interval La that is the misalignment amount when there is no decentering.
FIG. 7B illustrates the relationship between a distance from the rotary polygon mirror 210 to the aperture 207 and a deflection point interval on the rotary polygon mirror in a state where the distances between the rotary polygon mirror 210 and the A laser and B laser are fixed. On the left side in FIG. 7B, reference characters Ls denote a distance from the rotary polygon mirror 210 to the aperture 207, and on the right side in FIG. 7B, reference characters Ls′ denote a distance from the rotary polygon mirror 210 to the aperture 207. In this case the distance Ls is smaller than the distance Ls′. As shown in FIG. 7B, when the distance from the rotary polygon mirror 210 to the aperture 207 changes to the distance Ls′ from the distance Ls, the deflection point interval at the rotary polygon mirror 210 widens from the deflection point interval L2 to a deflection point interval L2′. To reduce main scanning jitter, it is necessary to reduce the deflection point interval L2 at the rotary polygon mirror 210 by decreasing a crossing angle that is formed by the beams of the A laser and B laser at the aperture 207. To achieve this, it is effective to bring the aperture 207 as close as possible to the rotary polygon mirror 210.